CS:S server doesn't require squat for CPU with humans only.
Bots however usually crush a server for some reason.
But an i7 should be able to handle a good number of bots without lag, so I'm not sure what the issue is.
You'll be fine with Humans though.
My Q6600 has about 60 Human L4D slots filled at any given time and it runs fine.

I personally wouldn't host a listen server (AKA "Host Server" or whatever it's called in game)